Microstructure devices often incorporate a thin film disposed atop a substrate surface, the film being formed into a cantilever, bridge, membrane, or other similar structure suspended over a cavity etched into the substrate. In some cases, the thin film may encase a sensing element, or may intrinsically provide a sensing function. For example, a thin film thermal conductivity sensor, used to detect the thermal conductivity of gas in the output flow from a gas chromatograph may include a thin film bridge of silicon nitride suspended over a cavity in a silicon substrate, the thin film encasing a metal film resistor and thereby providing environmental passivation to the resistor.
A thin film in a microstructure must resist mechanical and thermal shock, and must survive large temperature excursions during fabrication, storage, shipping, and use of the microstructure. However, differences in the coefficient of thermal of expansion (CTE) between the thin film and the substrate on which it is supported present a challenge to the thin film's survival.
Typically the thin film is formed, by some combination of deposition and growth, on a surface of the substrate at one or more formation temperatures, typically high temperatures, while the resultant device is stored, shipped, and operated over a temperature range which may go above or below the formation temperature. The thin film incorporates stresses which are intrinsic to the film at its formation temperature, and is subjected to additional thermal stress due to any thermal expansion mismatch between the substrate and the thin film, or due to any thermal expansion mismatch between materials within the thin film, or due to any thermal expansion mismatch between the thin film and any elements disposed within or upon the thin film. Further, the thin film is subject to environmentally-induced mechanical stress from sources such as shaking, dropping, air currents, water flow, and the like, depending on the particular environment. When some combination of intrinsic stress, thermal stress, and mechanical stress exceeds the tensile strength of the thin film, it is subject to destructive failure in the form of cracking or breaking.
For example, when the substrate comprises silicon with a CTE of 3.7 parts per million per degree Kelvin (CTESi=3.7 ppm/° K) and the film comprises silicon nitride, with a coefficient of expansion of CTEnitride=3.3 ppm/° K, the difference in CTEs between the thin film and the substrate is 0.2 ppm/° K, which difference can lead to mechanical failure of the film, either after the deposition when the film cools down, during shipping, or during the operation of the finished device. If the initial intrinsic stress in the film is zero at a deposition temperature of, for example, 300° C., and the substrate and thin film cool from the deposition temperature, to a room temperature of 25° C., the thin film could then be in a net state of compression. If a portion of the underlying support for the thin film is then removed to create a suspended structure, that structure is at risk of buckling at room temperature. The buckling may lead to localized rupture because the local bending involved in buckling creates localized tensile stress which exceeds the tensile strength of the thin film material. Further, if such a device is operated at a temperature higher than the deposition temperature, the thermal expansion mismatch can place the structure in a state of high tension, presenting a risk of rupture.